Yes folks, solid state can be fun, too!
Most of us are probably aware of the drawbacks of the classical OTL
tube circuit.
High output impedance, hard to drive output tubes, DC imbalance at the
speaker connections, many tubes in parallel, high tube wear etc.
Last week, I was fiddling around with an idea I had befo swapping
the output tube stage for a pushpull MOSFET stage.
I started out with the usual floating paraphase splitter with 12AX7,
followed by a common cathode driver stage with 12AU7, driving a stack
of IRF530 MOSFETS, connected in series with the drain out on the upper
half and the source on the lower half.
This worked, but not very good. DC shifts, temperature problems etc.
Then I realized I had some N- and P channel MOSFETS lying
around.........

So I changed the configuration to SRPP input with 6DJ8 and
2SK135/2SJ50 complementary pushpull with the sources at the output.
At the anode of the lower half of the SRPP, I attached 2 caps to drive
the gates, and applied bias with a floating ground.
This floating ground goes to a DC balancing servo circuit, keeping the
output at +- 5 mV DC.
It turned out that an idle current of 400 mA per MOSFET was allowed
for, thereby putting the output stage in class A up until 10 watts.
There's zero global feedback, though I might be temped to add a little
to lower the output impedance some more.
OTOH, adding some more MOSFETS might be an interesting idea, too.
The supply voltage is +- 25 V and rigid smoothing is needed to
attenuate ripple voltage on the supply lines.
The transformer is capable of delivering 10 amps per channel.

How does this sound? Well, it sounds like a good triode amp, only with
more "authority".
Bass control is better than with my KT88 PPP in triode amplifiers,
while the transparency is about the same.

On the test bench, it puts out nearly 20 Veff in 8 ohms, which equals
to 50 watts sine power, while the same voltage is maintained in 4
ohms.
An ideal companion to my Maggies !
Bandwidth is limited by a slewing network at the input to 200 kHz,
which is adequate.
THD is under 0,1 % near full power, and even lower at 1 watt.

In short, I'm very happy with this new contraption and though the idea
might not be new, it certainly is worth a try.

Next move will be a neat cabinet and some additional MOSFETs in
parallel, but I don't have the 2SK135/2SJ50 anymore, and apparently,
they're getting rare and expensive.
Does anyone know of a substitute for these transistors?
They don''t neccesarily have to have TO3 housings, I should be working
on the heat problem anyway. HUGE heatsinks are needed!

--
Sander deWaal
Vacuum Audio Consultancy

I said:


Yes folks, solid state can be fun, too!

Sorry, wrong group. Should have been RATubes.

--
Sander deWaal
Vacuum Audio Consultancy

In article ,
Sander deWaal wrote:

I said:


Yes folks, solid state can be fun, too!

Sorry, wrong group. Should have been RATubes.

You can't talk audio on RAO? Sounds like you're having fun!

Stephen

François Yves Le Gal said:

I don't have the 2SK135/2SJ50 anymore, and apparently,
they're getting rare and expensive.
Does anyone know of a substitute for these transistors?

The Hitachi metal TO3 versions have long been discontinued. You may consider
using the 2SK1058/2SJ162 which are nearly identical, except that they come
in TO3P plastic packages; get their "reissues" from Semelab; or pick the
recent BUZ900/BUZ905 from Magnatec (a Semelab sister company).

Thanks FY, I'll have a look at them!

--
Sander deWaal
Vacuum Audio Consultancy

MINe 109 said:

Sorry, wrong group. Should have been RATubes.

You can't talk audio on RAO? Sounds like you're having fun!

Oh man, I'm having fun indeed!
Few things are more valuable than to roll your own stuff.

--
Sander deWaal
Vacuum Audio Consultancy